Miscarriage - Causes of miscarriage

There are many reasons why a miscarriage may happen, although the cause is often not identified.

If a miscarriage happens during the first trimester of pregnancy (the first three months), it is usually due to problems with the unborn baby (foetus). About three in every four miscarriages happen during this period.

If a miscarriage happens during the second trimester of pregnancy (between weeks 14 and 26), it may be the result of an underlying health condition in the mother.

These late miscarriages may be caused by an infection around the baby which leads to the bag of waters breaking before any pain or bleeding, In rare cases, they can be caused by the neck of the womb opening too soon.

First trimester miscarriages

Most first trimester miscarriages are caused by problems with the chromosomes of the foetus.

Chromosome problems

Chromosomes are blocks of DNA. They contain a detailed set of instructions that control a wide range of factors, from how the cells of the body develop to what colour eyes a baby will have.

Sometimes, something can go wrong at the point of conception and the foetus receives too many or not enough chromosomes. The reasons for this are often unclear, but it means that the foetus will not be able to develop normally, resulting in a miscarriage.

It is estimated that up to two thirds of early miscarriages are associated with chromosome abnormalities.

Placental problems

The placenta is the organ that links the mother’s blood supply to her baby’s. If there is a problem with the development of the placenta, it can also lead to a miscarriage.

Things that increase your risk

An early miscarriage may happen by chance. However, there are several things known to increase your risk of problems happening.

The age of the mother has an influence:

in women under 30, 1 in 10 pregnancies will end in miscarriage
in women aged 35-39, up to 2 in 10 pregnancies will end in miscarriage
in women over 45, more than half of all pregnancies will end in miscarriage

Other risk factors include:

obesity
smoking during pregnancy
drug misuse during pregnancy
drinking more than 200mg of caffeine a day (one mug of tea contains around 75mg of caffeine, and one mug of instant coffee contains around 100mg of caffeine)
drinking more than two units of alcohol a week: one unit is half a pint of bitter or ordinary strength lager, a small glass of wine or a 25ml measure of spirits

Second trimester miscarriages

Long-term health conditions

Several long-term (chronic) health conditions can increase your risk of having a miscarriage in the second trimester. These are:

diabetes (if it is poorly controlled)
severe high blood pressure
lupus (a condition where the immune system attacks healthy tissue)
kidney disease
an overactive thyroid gland
an underactive thyroid gland
coeliac disease (having an adverse reaction to gluten)

Infections

The following infections may also increase your risk:

Medicines

Medicines increasing your risk include:

misoprostol (used for conditions such as rheumatoid arthritis)
retinoids (used for eczema and acne)
methotrexate (used for conditions such as rheumatoid arthritis)
non-steroidal anti-inflammatory drugs (used for pain and inflammation)

To be sure that a medicine is safe in pregnancy, always check with your doctor, midwife or pharmacist before taking it.

Read more about medicines during pregnancy.

Womb structure

Problems and abnormalities with your womb can also lead to second trimester miscarriages. Possible problems include:

non-cancerous growths in the womb called fibroids
an abnormally shaped womb

Weakened cervix

In some cases, the muscles of the cervix (neck of the womb) are weaker than usual. This is known as a weakened cervix or cervical incompetence. A weakened cervix may be due to a previous injury to this area usually following a surgical procedure.

The muscle weakness can cause the cervix to open too early during pregnancy leading to a miscarriage.

Polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a condition where the ovaries are larger than normal. It can lead to hormonal imbalances inside the womb.

Polycystic ovary syndrome is known to be a leading cause of infertility. There is some evidence to suggest it may also be linked to an increased risk of miscarriages in fertile women. However, the exact role polycystic ovary syndrome plays in miscarriages is unclear.

Misconceptions about miscarriage

An increased risk of miscarriage is not linked to:

a mother’s emotional state during pregnancy, such as being stressed or depressed
having a shock or fright during pregnancy
exercise during pregnancy (but discuss with your GP or midwife what type of exercise is suitable for you during pregnancy)
lifting or straining during pregnancy
working during pregnancy
having sex during pregnancy